1. Field of the Invention
Embodiments of the invention relate to control circuits of quasi-resonant switching power supplies.
2. Description of the Related Art
In a quasi-resonance switching power supply, a switching frequency increases with decrease in the load. As a result, a switching loss in a power semiconductor device increases in a light load condition, diminishing a conversion efficiency. In a condition of a load below 50% of the rated load in particular, the conversion efficiency decreases remarkably. The increased loss causes overheating of the power semiconductor device.
To cope with this problem, a technique of bottom skip has been proposed to control steady increase in the operating frequency in the light load condition.
The bottom skip control technique utilizes a resonant waveform at a drain terminal of the power semiconductor device, a switching element, such as a MOSFET or an IGBT. In a heavy load condition, the switching element is turned ON at a time of the first bottom of the resonant waveform, while in a light load condition, the switching element is turned ON at a time of a bottom later than the first bottom, which is a bottom skip mode.
The number of bottom skips in this bottom skip control is determined larger under a lighter load condition to restrain the increase in the switching frequency under a light load condition.
This bottom skip control reduces the switching frequency in a light load condition. Since simple decrease in switching frequency lowers the output voltage of a power supply, a means is needed to stably maintain the output voltage of the power supply in which ON time ratio is kept by elongating the ON width of the switching element to compensate for the decrease in the switching frequency. However, if the ON width is elongated in the bottom skip control that detects the load condition based on the ON width of the semiconductor element, the judgment of the load condition is not appropriate and so the number of bottom skip would return back to the previous value in the next switching cycle, which results in disturbance in the switching frequency and occurrence of an acoustic noise.
A Japanese translation of PCT International Application No. 2005-503748 discloses a technique to prevent the disturbance in the switching frequency. The technique sets, by timers, two periods of time corresponding to an upper limit and a lower limit of switching frequency fhi and flo, respectively, as shown in FIG. 12. In order to conduct switching operation within the period of time determined by the upper and lower limit frequencies, the number of bottom skips is set in a logic circuit such as an up/down counter.
When the load becomes light and the switching frequency increases over the upper limit fhi, the number of bottom skips is increased; when the load becomes heavy and the switching frequency decreases below the lower limit flo, the number of bottom skips is decreased. The actual number of bottoms in the resonant waveform is counted by a separate counter. When the actual number of bottoms equals the predetermined number of bottom skips, the switching element is turned ON.
When the setting of the number of bottom skips is changed to a one skip larger number and the switching frequency has decreased, the range of switching frequencies is determined so that the new switching frequency is higher than the lower limit frequency flo. On the other hand, when the setting of the number of bottom skips is changed to a one skip smaller number and the switching frequency has increased, the range of switching frequencies is determined so that the new switching frequency is lower than the upper limit frequency fhi.
Such setting of the upper and lower limits of switching frequencies with hysteresis prevents the switching frequency from disturbance in the process of bottom skip change over and avoids the acoustic noise.
U.S. Pat. No. 7,795,851 discloses a PWM power supply with a fixed frequency in which, in an event of decreased load, the switching frequency is decreased by means of a control voltage corresponding to the load. This PWM power supply reduces a switching loss in a light load period and improves efficiency of the power supply.
From the viewpoint of improving the efficiency in a light load condition, it is desirable that the switching frequency is reduced to a frequency of about 25 kHz a little higher than an audible frequency when the load has decreased to a specified magnitude and, when the load decreases further to a frequency lower than the specified magnitude, the operation is preferably shifted to a burst operation, which is a known technique to conduct switching operation at a high switching frequency in the period following a period without switching.
In the quasi-resonant control as described above, the number of bottom skips is set in a logic circuit such as an up/down counter and the bottoms of the actual resonant voltage is countered, and when the counted number of bottoms equals the set number of bottom skips, the switching element is turned ON. To achieve the decrease in the frequency down to about 25 kHz, the bottoms need to be counted at least ten times considering the period of resonant generally in the range of from 1 μs to 3 μs.
However, resonant oscillation after complete release of energy from the transformer damps with proceeding of the oscillation. Thus, the detectable number of bottoms may decrease below ten times. If the necessary number of bottoms cannot be detected, the bottom skip control collapse and the frequency cannot be decreased to a desired value.
In the example of FIG. 13 with a resonant period of 1.2 μs, the resonant voltage Vds damps generating about 15 bottoms. But, stable detection of bottoms is possible only to about 12th bottom. So, the resonant oscillation period is actually 1.2 μs×12=14.4 μs. Adding an ON time 1 μs of the switching element, which is a period of zero volts of Vds, and a flyback period 2 μs in which the resonant voltage Vds is fixed at a high level, to the resonant oscillation period 14.4 μs, the total period amounts to 17.4 μs. Thus, the switching frequency in this example can decrease only to 57.5 kHz.
The amplitude and period of the resonant oscillation could be made large by increasing the capacitance of the resonant oscillation capacitor connected in parallel to the switching element. However, this necessarily increases the drain capacitance of the switching element and tends to increase the switching loss in one time of switching. Therefore, the efficiency in the light load condition is deteriorated. Thus, as is described above, there is a need in the art for an improved control circuit of a quasi-resonant switching power supply.